The compound 5-[(1Z,2E)-2-methyl-3-phenylpropenylidene]-4-oxo-2-thioxo-3-thiazolidineacetic acid (shown below), referred to by its common name “epalrestat,” is a known active pharmaceutical ingredient (“API”) having beneficial therapeutic activity, for example as an aldose reductase inhibitor:

The preparation and pharmacologic activity of epalrestat is described in U.S. Pat. No. 4,831,045. Epalrestat is useful in treating and/or preventing various conditions such as, for example, complications of diabetes, as well as affording cardioprotection in non-diabetic patients. For example, epalrestat has a positive indication for the treatment and/or prevention of diabetic neuropathy, and is useful for the treatment and/or prevention of various other diabetic complications including, for example, diabetic retinopathy, diabetic nephropathy, diabetic cardiomyopathy, diabetic gastroparesis, diabetic microangiopathy, and diabetic macroangiopathy in mammals. Epalrestat is also useful in affording cardioprotection to subjects who may not be suffering from diabetes, and as a neuroprotectant or treatment for neurological or neurodegenerative disorders. Therapeutic activity of epalrestat in various conditions has been demonstrated in the clinical literature, including but not limited to Machii H. et al., Gendai Iryo, 1996; 28:1273; Miyamoto S. et al., Gendai Iryo, 1986; 18 (Extra Issue III):82; Goto Y. et al., Journal of Clinical and Experimental Medicine, 1990; 152:405; Nakano K. et al., Journal of Clinical and Experimental Medicine, 1990; 152:137; Okamoto H. et al., Internal Medicine, 2003; 42:655-664; Hamada Y. et al., Diabetes Care 2000; 23:1539-44; Goto Y. et al., Diabet Med 1993; 10(suppl 2):S39-43; Goto Y. et al., Biomed Pharmacother 1995; 49:269-77; Uchida K. et al., Clin Ther 1995; 17:460-6; Hotta N. et al., J Diabetes Complications 1996; 10:168-72; Hotta N. et al., Diabetes Care 2006; 29:1538-44; Matsuoka K. et al., Diabetes Res Clin Pract 2007; 77(suppl 1):S263-8; Nakayama M. et al., Diabetes Care 2001; 24:1093-8; Baba M., Journal of the Peripheral Nervous System 2003; 8:170; Yasuda H. et al., Diabetes Care 2000; 23:705; Ikeda T et al., Diabetes Research and Clinical Practice 1999; 43:193-198; Katayama M. et al., Electroencephalography and Clinical Neurophysiology/Electromyography and Motor Control 1995; 97:81; and Misawa S. et al., Neurology 2006; 66:1545-9.
Although therapeutic efficacy is a primary concern for a therapeutic agent, such as epalrestat, the salt and solid state form (e.g. crystalline or amorphous forms) of a drug candidate can be important to its pharmacological properties and to its development as a viable API. For example, each salt or each crystalline form of a drug candidate can have different solid state (physical and chemical) properties. The differences in physical properties exhibited by a particular solid form of an API, such as a cocrystal, salt, or polymorph of the original compound, can affect pharmaceutical parameters of the API. For example, storage stability, compressibility and density, all of which can be important in formulation and product manufacturing, and solubility and dissolution rates, which may be important factors in determining bioavailability, may be affected. Because these physical properties are often influenced by the solid state form of the API, they can significantly impact a number of factors, including the selection of a compound as an API, the ultimate pharmaceutical dosage form, the optimization of manufacturing processes, and absorption in the body. Moreover, finding the most adequate form for further drug development can reduce the time and the cost of that development.
Obtaining pure crystalline forms, then, is extremely useful in drug development. It may permit better characterization of the drug candidate's chemical and physical properties. For example, crystalline forms often have better chemical and physical properties than amorphous forms. As a further example, a crystalline form may possess more favorable pharmacology than an amorphous form, or may be easier to process. It may also have better storage stability.
One such physical property which can affect processability is the flowability of the solid, before and after milling. Flowability affects the ease with which the material is handled during processing into a pharmaceutical composition. When particles of the powdered compound do not flow past each other easily, a formulation specialist must take that fact into account in developing a tablet or capsule formulation, which may necessitate the use of additional components such as glidants, including colloidal silicon dioxide, talc, starch, or tribasic calcium phosphate.
Another solid state property of a pharmaceutical compound that may be important is its dissolution rate in aqueous fluid. The rate of dissolution of an active ingredient in a patient's stomach fluid may have therapeutic consequences since it can impact the rate at which an orally administered active ingredient may reach the patient's bloodstream.
Another solid state property of a pharmaceutical compound that may be important is its thermal behavior, including its melting point. The melting point of the solid form of a drug is optionally high enough to avoid melting or plastic deformation during standard processing operations, as well as concretion of the drug by plastic deformation on storage (See, e.g., Gould, P. L. Int. J. Pharmaceutics 1986 33 201-217). It may be desirable in some cases for a solid form to melt above about 100° C. For example, melting point categories used by one pharmaceutical company are, in order of preference, +(mp>120° C.), 0 (mp 80-120° C.), and −(mp<80° C.) (Balbach, S.; Korn, C. Int. J. Pharmaceutics 2004 275 1-12).
Active drug molecules may be made into pharmaceutically acceptable salts for therapeutic administration to the patient. Crystalline salts of a drug may offer advantages over the free form of the compound, such as improved solubility, stability, processing improvements, etc., and different crystalline salt forms may offer greater or lesser advantages over one another. However, crystalline salt formation is not predictable, and in fact is not always possible. Moreover, there is no way to predict the properties of a particular crystalline salt of a compound until it is formed. As such, finding the right conditions to obtain a particular crystalline salt form of a compound, with pharmaceutically acceptable properties, can take significant time and effort.
It is also possible to achieve desired properties of a particular API by forming a cocrystal of the API itself, or of a salt of the API. Cocrystals are crystals that contain two or more non-identical molecules. Examples of cocrystals may be found in the Cambridge Structural Database. Examples of cocrystals may also be found at Etter, M. C., and Adsmond, D. A., J. Chem. Soc., Chem. Commun. 1990 589-591; Etter, M. C., MacDonald, J. C., and Bernstein, J., Acta Crystallogr., Sect. B, Struct. Sci. 1990 B46 256-262; and Etter, M. C., Urbańczyk-Lipkowska, Z., Zia-Ebrahimi, M., and Panunto, T. W., J. Am. Chem. Soc. 1990 112 8415-8426, which are incorporated herein by reference in their entireties. The following articles are also incorporated herein by reference in their entireties: Görbotz C. H., and Hersleth, H. P. Acta Cryst. 2000 B56 625-534; and Senthil Kumar, V. S., Nangia, A., Katz, A. K., and Carrell, H. L., Crystal Growth & Design, 2002 2 313-318.
By cocrystallizing an API, or a salt of an API, with a co-former (the other component of the cocrystal), one creates a new solid state form of the API which has unique properties compared with existing solid forms of the API or its salt. For example, a cocrystal may have different dissolution and/or solubility properties than the active agent itself or its salt. Cocrystals containing APIs can be used to deliver APIs therapeutically. New drug formulations comprising cocrystals of APIs with pharmaceutically acceptable co-formers may, in some cases, have superior properties over existing drug formulations. However, cocrystal formation is not predictable, and in fact is not always possible. Moreover, there is no way to predict the properties of a particular cocrystal of a compound until it is formed. As such, finding the right conditions to obtain a particular cocrystal of a compound, with pharmaceutically acceptable properties, can take significant time, effort, and resources.
A crystalline form of a compound, a crystalline salt of the compound, or a cocrystal containing the compound or its salt form generally possesses distinct crystallographic and spectroscopic properties when compared to other crystalline forms having the same chemical composition. Crystallographic and spectroscopic properties of a particular form may be measured by XRPD, single crystal X-ray crystallography, solid state NMR spectroscopy, e.g. 13C CP/MAS NMR, or Raman spectrometry, among other techniques. A particular crystalline form of a compound, of its salt, or of a cocrystal, often also exhibits distinct thermal behavior. Thermal behavior can be measured in the laboratory by such techniques as, for example, capillary melting point, TGA, and DSC.
In the following description, various aspects and embodiments of the invention will become evident. In its broadest sense, the invention could be practiced without having one or more features of these aspects and embodiments. Further, these aspects and embodiments are exemplary. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practicing of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.